Blasting caps



Sept. 18, 1956 C. M. BLACKBURN BLASTING CAPS Filed May 25, 1951 #WWP/366345.

United States nLnsrINo `Cars This invention relates generally to blasting caps, and particularly to blasting caps of the character wherein an electrically energized element may be subject to mechanical abuse during the process of manufacturing the IIt has heretofore been proposed to utilize a plug of soft material, such as rubber, neoprene, or other rubberlike materials, as a closure about the lead wires at the open end of a blasting cap. In the process of crimping the open end of the blasting cap about such a soft plug, the latter is subject to considerable distortion and if, as is common, the lead Wires extending therethrough are joined at their inner tips by a so-called bridge wire, the latter is liable to be broken, bent, or otherwise damaged during the crimping operation, and such damage being concealed Within the cap, cannot be visually ascertained. Moreover, the distortion of the soft plug may be so severe that the inwardly projecting tip of a lead wire may be moved into proximity with the internal wall of the case where friction might be created, which would accidentally discharge thecap. Moreover, if the end of a lead wire be distorted to a lesser degree, but nonetheless such as to reduce the air gap between the tip of a lead wire and the interior of the case, the cap is rendered more subject to discharge by static electricity than if the tips were properly positioned within the cap. While certain features of the present invention are applicable to blasting caps wherein the closure of the case is effected by means other than a soft plug of the character aforesaid, the primary object of the invention is to protect caps provided with such soft closure plugs against the adversities above-mentioned.

In particular, the object of the invention is to provide a convenient and economical manner of maintaining the preferred spacing of lead wires with reference to each other and with reference to the case of an electric blasting cap.

Another object of the invention is to provide an electric blasting cap which is protected against discharge by static electricity. Y

Other objects will become apparent to those skilled in the art when the following description is read in connection with the accompanying drawings, in which:

Figure l is a perspective view of an assembly of lead wires, bridge wire, support, and closure plug for an electric blasting cap, all constructed in accordance with the present invention; v

Figure 2 is a sectional view taken along line 2 2 of Figure l;

Figure?) is a longitudinal sectional view of a blasting cap constructed in accordance with the present invention, but before being crimped; and

Figure 4 is a view in side elevation of the completed cap'shown in Figure 3, part being broken away to reveal the relationship of the elements, and part being shown in section. Y

In accordance with the present invention, generally stated, the lead wires of an electric blasting cap are sand- 2,763,21i Patented Sept. V18, 1956 wiched between two sheets of plastic material, which sheets are autogenously connected together about said lead wires, thus to maintain the latter in properly spaced relationship and relative orientation. It is preferable that the sheets of plastic material, partially or wholly overlap the insulation upon the lead wires, thereby to provide an anchorage inhibiting the removal of the plastic sheets lengthwise of the wires. At one side of the autogenously connected plastic sheets (hereinafter called the lead wire support), the bare terminals of the lead Wires are connected by a bridge wire. Preferably, such bare terminals extend for a relatively slight distance, for example, on the order of one-tenth of `an inch, beyond the lead wire support, so that there is relatively little likelihood for flexing of the projecting tips of the lead wires.

The aforesaid bridge wire support extends diametrically of the blasting cap, so as to approach closer to the interior walls of the case, than does the bridge wire or the exposed terminals of the lead wires. If the clearance between the longitudinal edges of the lead wire support and the interior of the case is on the order of about ten to fifteen thousandths of an inch or less, the assembly may be readily inserted into the open end of a blasting cap with assurance that the bridge wire and the exposed terminals of the lead wires cannot move relative to eachother and must be located substantially near the longitudinal axis of the cap.

ln case it is desired to provide added assurance against discharge of the cap by static electricity, the interface between the respective sheets of plastic may be coated with an electrically conductive material of relatively high resistance so as to make electrical Contact with at least one of the lead wires and extend fairly close to the interior wall of the case.

Having applied the lead wire support to the lead wires, the bridge wire may be readily connected in any convenient way, the closure plug may be applied, and the case crimped, without subjecting the bridge wire to mechanical strains or distortion.

Referring now to the drawings for an illustrative embodiment of the invention, the blasting cap consists of the usual metallic cup 1, the closed end of which is provided with a suitable train of explosive charges including, for example, a base charge 2, an initiator charge 3, relay charge 4, and an ignition charge 5. In accordance with a common practice, the charge 5 is ignited by electrical energization of a bridge wire 6, which is connected across the ends of a pair of lead wires 7 and 8. Instead of a multiplicity of explosive charges such as 2, 3, 4, and 5 which, for example, might be used-in a compound detonator, a lesser number or only one explosive charge may be used in accordance with this invention depending upon the product desired.

The bridge wire 6 is soldered or otherwise electrically and mechanically connected to each of lead wires 7 and S. In order to facilitate the connection of the bridge wire 6 to the lead wires, to maintain the lead wires 7l and 8 in proper spaced relation adjacent the bridge wire, and thus prevent mechanical damage to the bridge wire pursuant to relative'movement of the lead wires 7 and 8 thereadjacent, the present invention contemplates that said lead wires be sandwiched .between two strips of thermoplastic material 9 and 10, which strips are autogenously connected together about and beyond the respec tive lead wires so that strip 9 is tangent with both lead wires through substantially half of their circumferences, and strip l0 is tangent with both lead wires through vsubstantially the opposite half of their circumferences. In the application of the strips 9 and l@ to the lead wires it is preferable that the strips 9 and 1t) be arrangedto extend outwardly beyond the lead wires 7 and S, so that the lends thereof approach closer to the interior of case 3 1 than do the tips of the lead wires 7 and 8 or the bridge wire 6.

The strips 9 and 10 are"`preferably of thermoplastic material, as such a material canreadily, with the aid uof heat and pressure, be conformed about the lead wires 7 and 8, and the respective strips autogenously connected together. In the embodiment shown, the respective strips 9 and 1() have a thickness on the order of 0.030 inch, and may be formed of a vinyl resin (such as that known as Vinylite), an ethyl cellulose (such as the composition known as `Ethocel), or any other thermoplastic resin composition which is available in sheet form and which when set has substantial resistance to mechanical ilexing. The` sheets 9 and 10 may, however, be formed of an uncured thermosetting resin provided in the form of readily flexible sheets which, when subjected to heat and pres sure,.set into conformity with, and into autogenous connection about, the lead wires 7 and 8, as for example, a phenol formaldehyde resin or a rigid thermosetting polyester resin may be used for the purpose. If desired the stripsr 9 and 10 may be a single piece folded double about the lead wires, or` they may be formed of such a resin or adhesive backed by paper, cardboard, or other sufficiently rigid material.

In applying the sheets 9 and 10 to the lead Wires 7 and 8, it is preferable that said sheets 9 and 10 be ar ranged so as to be tangent respectively with diametrically opposite cylindrical surfaces of the lead wires in a position where the sheets overlap, wholly or partially, the insulation upon the lead wires 7 and 8, as thereby the anchorage of the sheets 9 and 10 is rendered secure against longitudinal displacement along the lead wires. Particularly good anchorage between the sheets 9 and 10 is rendered secure against longitudinal displacement along the lead wires. Particularly good anchorage between the sheets 9 and 10 and the lead wires is secured when the insulation upon the lead wires 7 and 8 is a thermoplastic material, because thereby when heat and pressure are applied to the sheets 9 and 10, said sheets become interfused with the thermoplastic insulation on the lead wires. In the` embodiment shown, the strips 9 and 10 partially overlap the insulation 11 on the lead Wires.

Where the strips 9 and 10 are of Ethocel, they may be preheated to soften them and render them readily conformable to the contour of the lead wires. Preheating at a temperature of 250 degrees F. for a period of l5 seconds is sufficient for the purpose or, if desired, the surfaces may be softened by the application of a suitable solvent. With the strips 9 and .10 thus softened, they are applied to the lead wires as above described, pressed into contact with each other, and cooled to set them. Alternatively, the strips 9 and 10 may be concurrently heated and pressed into position on the lead wires as by the use of a die, maintained 'at a temperaturev of 280 degrees F. vIn the latter instance, the assembly is removed from the die while soft and set by cooling in the atmosphere.

The lead wire support above described lends itself particularly to the protection of the ignition composition from discharge by static electricity. Where it is desired to accomplish such result, the interface between the strips 9 and 10 may be provided with a layer 12 of electrically conductive material (having a high electrical resistance) such as a metal base pigment. Suitable pigments are acetylene black, graphite, tine aluminum or the like. Such pigments may be applied as a thin lm of paint (using a vehicle which readily wets the material of sheets 9 and 10) to the face of one or both of strips 9 and 10, which will constitute the interface between them when autogenously connected. With such a layer of electrically conductive material interposed between the sheets 9 and 10 in electrical contact with the lead wires 7 and 8, and extending outwardly therebeyond so as to be closer to the inside of the case 1 than is the bridge wire 6, it is apparent that opposite charges of electricity on the case fi and lead wires will discharge across the space between the ends of sheets 9 and 10 and the interior of the case rather than from the exposed tip of the lead Wires 7 and 8 (through the ignition mixture) to the interior of the case.

The lead wire support hereinbefore described is particularly advantageous where the closure plug 13 for the cap is formed of relatively soft material, such as rubber or neoprene. In such cases, it is customary to crimp the metallic case about the closure plug, as shown at 16, and in the course of so doing, the plug is distorted, and the relative position of the lead wires 7 and 8 therein altered. Such may result in the breakage of or other damage to the bridge wire 6, or may result in moving the exposed tips of the lead Wires into proximity with the interior of the case, where friction might create an explosion. The provision of the lead wire support above described, particularly whenarranged sothat the bare tips of lead wires 7 and 8 are relatively short (i. e. with the axial space between the bridge Wire and the nearest edge of the support, less than the radial space between the bare tips of the lead wires) substantially eliminates these diiculties.

In the embodiment shown, the plug 13 is provided with a pair of slits 14 extending axially thereof in order to facilitate the insertion of the lead wires through preformed holes in the plug 13; and furthermore, the plug 13 is provided with a notch 15 sized to accommodate the support composed of sheets 9 and 10, but these features are the inventions of Frederick R. Seavey and Thomas I. Mulqueeny. From the foregoing description, it should be apparent to those skilled in the art that the lead wire support of the present invention whenused in combination with blasting caps having relatively soft and exible closure plugs overcomes the disadvantages heretofore experienced with such plugs, and does so in a convenient and economic manner. It is evident, therefore, that the invention accomplishes its objects aforesaid.

While one complete embodiment of the invention has been disclosed in detail and various alternatives suggested, it is to be distinctly understood that the invention is not limited to the details of the foregoing disclosures, but

various modifications and variations may be made within the spirit of the invention without departing from the scope of the appended claims.

Having thus described the invention, what is claimed and desired to be secured by Letters Patent 1s:

1. In a blasting cap having a shell and a soft rubberlike plug closure about lead Wires and in contact with the interior of the shell adjacent the mouth thereof, thc combination of a llead wire support of relatively stiff ma- Iterial gripping said lead wires in xed spaced relation adjacent the inner end of said soft rubber-like plug closure, a 'bridge wire linterconnecting said lead wires adjacent the edge of said support, and said c'ase being crimped into deforming relationship with the soft rubberlike plug closure outwardly of said support.

2. In a blasting cap having a soft-plug closure about lead wires, a lead wire support maintaining said lead wires -in xed spaced relation adjacent the inner end ot said soft-plug closure, and a bridge wire interconnecting said lead Wires adjacent the edge of said support, the improvement which comprises, said lead wire support being composed of a pair of plastic strips -autogenousl'y connected together in tangential relationship respectively with oppositel surfaces 'of the lead Wires through substantially half of their circumferences 3. In a blasting cap having a soft-plug closure about lead wires, a lead Wire `support maintaining said lead wires in xed spaced relation adjacent the inner end of said soft-plug closure, and a` bridge Wire interconnecting said lead wires adjacent `the edge of said support,the improvement which comprises, said lead wire support being composed of a pair of plastic strips autogenously connected together about the respective lead wires and interfused with insulation on said lead wires.

4. In a blasting cap having a soft-plug closure about lead wires, a lead wire support maintaining said lead wires in fixed spaced relation ladjacent the inner end of `said sotdplug closure, and a bridge wire interconnecting said lead wires adjacent the edge of said support, the improvement whichcomprises, said lead Wire support being composed of a pair of plastic sti-.ips autogenously connected together about the respective lead wires With a lm of high resistance electrically conductive material therebetween and in contact with a conductive part of said lead wires.

5. In the manufacture of electric blasting caps, the method comprising, providing a pair of insulated lead Wires having bare tips, applying a strip of 'thermosensitive plastic to said wires adjacent said tips in tangential bridging relation thereto, conforming the strip about each of said wires so as to be tangent with both said lead wires through substantially half of their circumferences, and setting the strip in autogenous connection about said Wires while said strip is confonmed thereto.

6. The method of claim 5 wherein the wires are provided with thermoplastic insulation and the thermosensitive strip is interfused with said insulation.

7. The method of claim 5 wherein strips of thermosensitive plastic are applied on opposite sides of Ithe lead wires and autogenously 4connected together between and beyond the lead wires.

8. The method of claim 5 wherein Istrips of thermosensitive plastic are applied on `opposite sides of the lead wires with a lm of high resistance electrically conductivo material therebetween in electrical contact with a conductive part of the lead wires.

References Cited in the le of this patent UNITED STATES PATENTS 1,751,827 Marshall Mar. 25, 1930 2,133,119 Smith et al Oct. 11, 1938 2,191,544 Ruskin Feb. 27, 1940 2,240,438 Durant Apr. 29, 1941 2,382,921 Seavey Aug. 14, 1945 2,477,458 Johnson July 26, 1949 2,527,858 Stuart Oct. 3l, 1950 FOREIGN PATENTS 543,500 Great Britain Feb. 27, 1942 

